1. Field of the Invention
The present invention relates to an optical scanning device which is capable of scanning an object surface with a laser beam. More particularly, the present invention is concerned with a scanning type optical device suitable for use in a recording apparatus such as a laser beam printer.
2. Description of the Related Art
FIG. 10 illustrates a known optical scanning device capable of scanning an object surface with a laser beam. This optical scanning device has a beam source unit 100 including a laser source. An image exposure scanning operation is executed by applying the laser beam L' from the beam source unit 100 to a photosensitive member 105 through a scanning optical system 104 which includes a rotary polygonal mirror 102 and a group of lenses 103 and which is housed in a housing 101, whereby an image scan of the surface of the photosensitive member 105 is executed.
In order to obtain a high degree of accuracy of image formation on the photosensitive member 105, it is important that the beam source unit 100 and the components of the optical scanning system 104 such as the rotary mirror 102 and the group of lenses 103 are precisely located in relation to each other.
In addition, contamination of optical members such as the rotary polygonal mirror by dust and other foreign matter which may attach thereto should be avoided.
In order to achieve a high degree of scanning precision, the beam source unit 100 and other optical members are mounted on predetermined portions of a housing 101. The housing 101 has a cover portion (not shown) which effectively keeps dust and other foreign matter from internal optical parts.
The beam source unit 100 will be described in more detail.
As shown in FIG. 11, the beam source unit 100 includes a laser chip 106 and a collimator lens 107 which collimates the laser beam emitted from the laser chip 106 into a parallel beam and which is assembled together with the laser chip 106.
The beam source unit 100 is fit in a mounting hole 108 formed in a side wall 101a of the housing 101 from the exterior of the housing 101 so that the position of emission of the laser beam is precisely determined. In addition, an abutment member having an abutment surface 101b orthogonal to the laser beam axis is provided so as to locate and fix the beam source unit 100 in the direction of the optical axis.
In the known scanning device of the type described, it is necessary that the mounting hole 108 is formed to have a size which is slightly greater than the size of the beam source unit 100, in order to accommodate any tolerance in the machining of the hole 108, we well as any fluctuation in the size of the beam source unit 100 which may be incurred in the course of manufacture of the beam source unit 100.
It is therefore necessary to perform a time-consuming and high-precision step in order to ensure that the beam source unit 100 is correctly centered with respect to the mounting hole 108 so as to set the beam source unit 100 in correct optical alignment with the associated optical parts.
On the other hand, there is an increasing demand for mass-production of optical scanning system, to cope with current demand for laser beam printers and other apparatuses which incorporate this type of device.
The mass-production of optical scanning system is preferably conducted through an automatic assembly process which employs industrial robots. Such an automatic assembly process, however, encounters a difficulty due to the delicate and troublesome adjusting operation which is indispensable for correctly centering the beam source unit with respect to the mounting hole.
Usually, the housing 101 is formed by molding from a resin. The configuration of the housing 101 is complicated due to the presence of the mounting hole 108 and the abutment surface 101b for locating the beam source unit in the direction of the optical axis, so that it is not easy to mold the housing with a high degree of precision.
This imprecision may lead to such imprecision in the position of the beam source unit 100 that the laser beam L' running towards the rotary polygonal mirror is deviated from the designated optical path, with the result that the scanning beam is aimed at the wrong position or that the scanning line warps undesirably.
For these reasons, it is very important that the molding of the housing is conducted with the greatest care and high degree of precision.
The optical scanning device is usually located on an end portion of the apparatus which incorporates the scanning device. In addition, various components and parts are densely arranged around the optical scanning device in order to meet the requirement for a compact design of the apparatus.
When the laser chip of the beam source unit has been exhausted or becomes defective, it is necessary to demount the beam source unit and then mount a new beam source unit on the housing. This requires a laborious work for demounting and remounting various parts and components around the beam source unit.